1. Field of the Invention
The present invention relates to a sensor detection apparatus having a sensor detection circuit capable of detecting abnormality of a sensor element which uses bridge circuits and to a sensor provided with such a sensor detection apparatus.
2. Description of Related Art
FIG. 1 is a circuit diagram showing a configuration of a conventional acceleration sensor. In FIG. 1, reference numeral 100 denotes a sensor element which senses acceleration and reference numeral 110 denotes a sensor detection apparatus which detects the acceleration from an output of the sensor element 100. In the sensor element 100, four piezoresistors R1, R2, R3, and R4 whose resistance values vary according to pressure applied to themselves, form a Wheatstone bridge circuit. The piezoresistors R1 an R2 in series and the piezoresistors R3 and R4 in series are connected in parallel between a power source terminal 101 to which a driving power source is given, and a grounding terminal 104 connected to a ground potential. As outputs of the sensor element 100, a potential Vs+ between the piezoresistors R1 and R2 is outputted from a first output terminal 102 and a potential Vs− between the piezoresistors R3 and R4 is outputted from a second output terminal 103.
In a case where it is assumed that the piezoresistors R1, R2, R3, and R4 each have a resistance value of R, the potentials Vs+ and Vs− are equal when no acceleration is applied to the sensor element 100. However, when acceleration is applied, for example, the resistance values of the piezoresistors R1 and R4 on the diagonal position vary to R+ΔR and those of the piezoresistors R2 and R3 on the diagonal position as well vary to R−ΔR. As a result, a potential difference corresponding to ΔR is developed between the potentials Vs+ and Vs−.
The sensor detection apparatus 110 is provided with a power circuit 115 for driving the sensor element 100. The output of the power circuit 115 is outputted from a power output terminal 111, and given to the power terminal 101 of the sensor element 100. The two potentials Vs+ and Vs− of the sensor element 100 are inputted to an amplifier 116 in the sensor detection apparatus 110 via a first input terminal 112 and a second input terminal 113 of the sensor detection apparatus 110, respectively. The amplifier 116 amplifies the potential difference between the potentials Vs+ and Vs− and then outputs a result from an output terminal 114. Acceleration applied to the sensor element 100 can be judged based on the output from the output terminal 114.
In Japanese Utility Model Application Laid-Open No. 4-109371 (1992), a diagnostic circuit has been proposed which is able to detect failure such as a disconnection or a short circuit having occurred at a Wheatstone bridge circuit by monitoring whether two output potentials of the Wheatstone bridge circuit fall within a certain potential range with use of a window comparator. For example, when a disconnection has occurred at the Wheatstone bridge circuit, the output potentials become a power supply potential or a ground potential, so that the abnormality can be detected by the window comparator.
Also, in Japanese Utility Model Application Laid-Open No. 5-81720 (1993), a failure detection apparatus has been propose which is able to detect failure such as a disconnection or a short circuit having occurred at a Wheatstone bridge circuit by monitoring whether a potential difference between both ends of a failure detecting resistor, which is connected in parallel to the Wheatstone bridge circuit, falls within a certain range with use of a window comparator.
In Japanese Patent Application Laid-Open No. 2003-194646, a pressure measuring apparatus has been proposed which is able to reduce the effect of a transient phenomenon of a variation in the output of a Wheatstone bridge circuit and, therefore, increase detection accuracy by applying a pulse voltage to the Wheatstone bridge circuit as a power supply, sampling potential differences between two output voltages at constant time period intervals for measurement, and outputting measured values whose variation has decreased below a threshold value as the result of the measurement.
However, in a diagnostic circuit described in Japanese Utility Model Application Laid-Open No. 4-109371 (1992) and a failure detection apparatus described in Japanese Utility Model Application Laid-Open No. 5-81720 (1993), it is possible to detect a failure such as a disconnection or a short circuit which has occurred at a Wheatstone bridge circuit; however, there is the problem that it is impossible to detect a failure occurred at an amplifier which amplifies a potential difference between two outputs of the Wheatstone bridge circuit.
In particular, when an acceleration sensor and a sensor detection apparatus are comprised of different chips, an amplifier of the input stage of the sensor detection apparatus may break due to static electricity which enters at the input terminal of the IC of the sensor detection apparatus. When the amplifier has broken, it becomes impossible to correctly perform the detection of acceleration; therefore it is desirable that a configuration be provided in which a sensor detection circuit is able to detect the break in the amplifier.
Besides, since the potential difference between the two outputs of the Wheatstone bridge circuit is minute, noise brings about much effect. Therefore there is the problem that the accuracy in the detection deteriorates due to the noise. Moreover, as the amplifier which amplifies the potential difference, high-gain amplifiers are used often. Because of this, the offset of the amplifiers becomes larger, so that there is the problem that accuracy in the detection deteriorates. Japanese Utility Model Application Laid-Open No. 4-109371 (1992), Japanese Utility Model Application Laid-Open No. 5-81720 (1993), and Japanese Patent Application Laid-Open No. 2003-194646 do not refer to these problems.